macsec: Add support for GCM-AES-256 cipher suite

This adds support for the GCM-AES-256 cipher suite as specified in
IEEE 802.1AEbn-2011. The prepared cipher suite selection mechanism is used,
with GCM-AES-128 being the default cipher suite as defined in the standard.

Signed-off-by: Felix Walter <felix.walter@cloudandheat.com>
Cc: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>